For purposes of clarity, the term “conventional engine” as used in the present application refers to an internal combustion engine wherein all four strokes of the well-known Otto or Diesel cycles (the intake, compression, expansion and exhaust strokes) are contained in each piston/cylinder combination of the engine. Each stroke requires one half revolution of the crankshaft (180 degrees crank angle (CA)), and two full revolutions of the crankshaft (720 degrees CA) are required to complete the entire Otto or Diesel cycle in each cylinder of a conventional engine.
Also, for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine generally includes:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston is operable to reciprocate through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston is operable to reciprocate through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and
a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve operable to define a pressure chamber therebetween.
A split-cycle engine replaces two adjacent cylinders of a conventional engine with a combination of one compression cylinder and one expansion cylinder. The four strokes of the Otto or Diesel cycle are “split” over the two cylinders and such that the compression cylinder provides for the intake and compression strokes and the expansion cylinder provides for the expansion and exhaust strokes. The Otto or Diesel cycle is therefore completed in these two cylinders once per crankshaft revolution (360 degrees CA).
U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Carmelo J. Scuderi (the “Scuderi patent”) and U.S. Pat. No. 6,952,923 granted Oct. 11, 2005 to David P. Branyon et al. (the “Branyon patent”) each contain an extensive discussion of split-cycle and similar type engines. In addition, the Scuderi and Branyon patents disclose details of prior versions of engines of which the present invention comprises a further development.
Split-cycle engines typically rely on maintaining pressure in the crossover passage at a high minimum pressure (typically 20 bar or higher) during all four strokes of the Otto or Diesel cycle. Maintaining maximum pressure levels in the crossover passage generally results in the highest efficiency levels.
Also, spark-ignition (or Otto) split-cycle engines preferably maintain an appropriate mixture of air and fuel in the expansion cylinder prior to spark ignition. A stoichiometric air/fuel mixture (approximately 14.7 times the mass of air to fuel) is ideal. A rich mixture (less than approximately 14.7 times the mass of air to fuel) can leave excess fuel, which reduces efficiency. A lean mixture (more than approximately 14.7 times the mass of air to fuel) can produce too much nitrous-oxide (NOx) for a catalytic converter (not shown) to process, causing an unacceptable level of NOx emissions.
In prior art split-cycle engines, the XovrC valves, XovrE valves, and fuel injectors of each of the one or more crossover passages operate synchronously. In other words, if there are multiple crossover passages, the XovrC valves open and close at approximately the same time, the XovrE valves open and close at approximately the same time, and the fuel injectors inject approximately the same amount of fuel into their respective crossover passages at approximately the same time.
Spark-ignition (or Otto) split-cycle engines can control load by varying the mass of air entering the compression cylinder. This can be done by utilizing variable valve actuation of the intake valve, although a throttling valve may also be used. At part-load conditions, the intake valve of the compression cylinder typically closes as compression piston is in its downward stroke (i.e., when the compression piston is moving away from the cylinder head). The result is that the compression cylinder does not intake a full charge of air. In other words, under part-load conditions, the pressure in the compression cylinder when the compression piston is at its bottom dead center position is typically less than 1 atmosphere.
Controlling load by varying the mass of air entering the compression cylinder allows spark-ignition (or Otto) split-cycle engines to maintain an appropriate mixture of air and fuel in the expansion cylinder. However, controlling load in this manner may have adverse effects. In prior art split-cycle engines, compressing less than a full charge of air in the compression cylinder reduces the pressure in the one or more crossover passages because the same mass of air is not moved/compressed into the one or more crossover passages as is moved/compressed at full-load. This of course does not maintain the desired maximum pressure levels in the crossover passages and can reduce the pressure below the aforementioned high minimum pressure requirements of split-cycle engines (typically 20 bar or higher).
Accordingly, there is a need to meet the high minimum pressure requirements of one or more crossover passage of a split-cycle engine at part-load conditions. More particularly, there is a need to maximize the pressure in the one or more crossover passages of spark-ignition split-cycle engines operating at part-load.